The Übermacht Sentinel XS is a model of car that spawns in the map in a predictable manner and can be sold for a good price, which makes it possible to farm them and get significant profit.
In the method introduced by this article, two different variants of Sentinel XS can spawn: the yellow one is worth $14,970, and the purple one can be sold for $17,420.
Make sure the in-game time is 22:00-04:00. You may check this from your iFruit phone or in the top-right corner of the Pause Menu.
Circle the block shown in the map below in counter-clockwise direction using a vehicle, starting from wherever you want. The block is in the vicinity of Eclipse Tower and Eight-Bit Arcade.
When you turn onto the north side of the block and drive towards west, you might see a Sentinel XS in either yellow or purple coming to you in the opposite direction. It usually appears in front of the Eight-Bit Arcade. If you don’t see it, circle one more time. If it still does not spawn, leave the area and come back, then try again.
After getting the car, sell it at the nearest Los Santos Customs. If you have sold an NPC vehicle in the past 48 real-life minutes, you will see a message saying you can’t sell the Sentinel XS. Read the “Vehicle Selling Limits” section below for workarounds.
To prevent receiving a Bounty from NPCs due to robbing their cars, you can enable Passive Mode beforehand.
Presence of other players can interfere with spawning of Sentinel XS, so using an Invite Only Session for farming is strongly recommended. If you want to farm with your friends together, you may carpool when you circle the block.
You can only sell one car to Los Santos Customs per 48 real-life minutes. So, you may choose to store the cars that can’t be sold yet in your Personal Garage and sell them later.
Alternatively, you can also register as a VIP or CEO, hire Bodyguards or Associates, and let them sell vehicles for you. You can get 90% of the revenue, whereas the rest 10% goes to SecuroServ (deducted by Rockstar, actually). However, this would still activate the 48-minute timer for your Bodyguards or Associates.
Also, your Bodyguards or Associates cannot sell cars that have been stored in your Personal Garage. If you still have some Sentinel XS’s left in your Garage, it’s you who should sell them because it can only be done by you; let your Bodyguards or Associates sell the cars that have just been robbed on the street.